docs: def export_aggregate_by_csv(ekey, dstore):
      """
      :param ekey: export key, i.e. a pair (datastore key, fmt)
      :param dstore: datastore object
      """
      token, what = ekey[0].split('/', 1)
      aw = extract(dstore, 'aggregate/' + what)
      fnames = []
      writer = writers.CsvWriter(fmt=writers.FIVEDIGITS)
      path = '%s.%s' % (sanitize(ekey[0]), ekey[1])
      fname = dstore.export_path(path)
      writer.save(aw.to_table(), fname)
      fnames.append(fname)
      return fnames